The biggest issue I see with what you are trying to achieve is the angles at such a small distance from the target. Even if you use a 180 degree fisheye lens you will be looking down the sides of the truck at the edges of the image, and that may prevent you getting a clear view of the wheels, the spherical aberration will also make it difficult to gauge how long the truck is.
A camera placed at another point in the toll plaza that captures the side of the truck from further away may achieve what you are after.
I also wonder why you need to do this with CCTV. There are plenty of automatic systems that will count axles and determine vehicle length for toll systems.
Thanks for the suggestion. We thought of placing the Camera at different points. But, in our case Traffic is very crowded and other vehicles will obstruct the view. We want to eliminate the Loop base automatic counters and planning to move Camera based counters.
-Thanks.